Display device with improved contrast

ABSTRACT

A display device comprising: a light-emissive structure including two regions of light emissive material for emitting light in a viewing direction, the regions being spaced apart in a direction perpendicular to the viewing direction the light-emissive structure being capable of guiding light emitted from one of the light-emissive regions towards the other emissive region; and a barrier structure located between the light-emissive regions for inhibiting the propagation of light guided from the said one of the light-emissive regions to the other light-emissive region.

FIELD OF THE INVENTION

One specific class of display devices is those that use an organicmaterial for light emission. Light-emissive organic materials aredescribed in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents ofboth of which are incorporated herein by reference. The basic structureof these devices is a light-emissive organic layer, for instance a filmof a poly(p-phenylenevinylene (“PPV”), sandwiched between twoelectrodes. One of the electrodes (the cathode) injects negative chargecarriers (electrons) and the other electrode (the anode) injectspositive charge carriers (holes). The electrons and holes combine in theorganic layer generating photons. in PCT/WO90/13148 the organiclight-emissive material is a polymer. In U.S. Pat. No. 4,539,507 theorganic light-emissive material is of the class known as small moleculematerials, such as (8-hydroxyquinolino)aluminium (“Alq3”). In apractical device one of the electrodes is typically transparent, toallow the photons to escape the device.

BACKGROUND OF THE INVENTION

FIG. 1 shows the typical cross-sectional structure of an organiclight-emissive device (“OLED”). The OLED is typically fabricated on aglass or plastic substrate 1 coated with a transparent first electrode 2such as indium-tin-oxide (“ITO”). Such coated substrates arecommercially available. This ITO-coated substrate is covered with atleast a layer of a thin film of an electroluminescent organic material 3and a final layer forming a second electrode 4, which is typically ametal or alloy. Other layers can be added to the device, for example toimprove charge transport between the electrodes and theelectroluminescent material.

There are several approaches available for the processing of conjugatedpolymers such as PPV. One approach uses a precursor polymer which issoluble and can therefore be easily coated by standard solution-basedprocessing techniques. Examples of coating techniques include: spincoating, blade-coating, reverse roll coating, meniscus coating,contact/transfer coating, and ink-jet printing. The precursor is thenconverted in situ by suitable heat treatment to give the fullyconjugated and insoluble polymer. Another approach uses directly solubleconjugated polymers which do not require a subsequent conversion stage.Depending on the specific application, one or other of the approachesmight be preferred. The precursor polymer approach can be especiallyuseful where subsequent processing might lead to damage of the polymerfilm if it were directly soluble—such processing may be, for instance,coating with further polymer layers (for example, transport layers oremitting layers of another colour), or patterning of the top electrode.Converted precursor films also have better thermal stability, which isof importance both during fabrication and for the storage and operationof devices at high temperatures. FIG. 2 illustrates one arrangement fordepositing light-emissive polymers by ink-jetting, where a glass sheet10 is coated with an electrode 11 and light-emissive material 12 canthen be deposited by ink-jetting on to the electrode 11. A secondelectrode can then be deposited over the light-emissive material. (See,for example PCT/WO98/24271, the contents of which are incorporatedherein by reference).

When light is produced in an electroluminescent display or other lightemitting device it is emitted in all directions. In a device of the typedescribed above some light is emitted forwards, in a viewing direction,through the transparent electrode to the viewer, whilst some is emittedbackwards to the opaque metallic electrode where it is either reflectedforwards to the viewer or absorbed. Another portion of the light, theportion that is emitted or scattered to more oblique angles, can bewaveguided within the emissive layer or within other layers such as thetransparent electrode or charge transport layers. The part of thewaveguided light that is not absorbed can eventually reach the edge ofthe emissive pixel. This light is travelling in a direction roughlynormal to the principal viewing direction and will not contribute to thebrightness of the device as seen by the viewer (see N. C. Greenham etal., Advanced Materials 6 (1994) p491).

The optical structure of the device, and specifically the thicknessesand refractive indices of the component layers, plays an important rolein determining how efficiently it is possible for emitted light to becontained within the plane of the device, and thus move away from theelectrically-driven pixel. For example, it is possible for light to be‘trapped’ in ‘slab waveguide’ modes which propagate within the plane ofa device of the type shown in FIG. 1. A general condition forwaveguiding in a region of material is that the region should have ahigher refractive index than the materials on either side of it. Theemissive organic semiconducting layers can themselves act as this higherrefractive index region, in which waveguiding can occur, since thesematerials commonly show higher refractive indices than the opticallytransparent materials, such as inorganic glasses or organic polymerswhich are used as substrate, cladding or insulating layers. Theoccurrence of this type of waveguiding has been described in some detailin the context of optically-stimulated gain in structure made with suchmaterials, as described for example in: “Spectral Narrowing inOptically-Pumped Poly(p-phenylenevinylene) films”, G. J. Denton, N.Tessler, M. A. Stevens and R. H. Friend., Adv. Mater. 9, 547-551 (1997),“plastic lasers: comparison of gain narrowing with a solublesemiconducting polymer in waveguides and microcavities”, M. A. DiazGarcia, F. Hide, B. J. Schwartz, M. D. Mcgehee, M. R. Andersson and A.J. Heeger, Appl. Phys. Lett. 70, 3191-3193 (1997), and “Lightamplification in organic thin films using cascade energy transfer”, M.Berggren, A. Dodabalapur, R. E. Slusher and Z. Bao, Nature 389, 466-468(1997).

One common type of display comprises an array of light-emissive regionsthat can be controlled as independent pixels to allow a desired patternto be displayed. The array is normally planar, with the light-emissiveregions and their associated electrodes and other circuitry formed on asubstrate such as a glass sheet. In a device of this type the obliquelyemitted light travels in a direction generally in the plane of thedisplay. In a multi-pixel device the waveguided light can cause furtherproblems by causing cross-talk between pixels and reducing the contrastbetween emitting and non-emitting pixels.

The present invention aims to at least partially address one or more ofthese problems.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided adisplay device comprising: a light-emissive structure including tworegions or light-emissive material for emitting light in a viewingdirection, the regions being spaced apart in a direction perpendicularto the viewing direction and the light-emissive structure being capableof guiding light emitted from one of the light-emissive regions towardsthe other emissive region; and a barrier structure located between thelight emissive regions for inhibiting the propagation of light guidedfrom the said one of the light-emissive regions to the otherlight-emissive region.

The barrier structure may be a light-absorbent barrier structure of alight-reflective barrier structure. The barrier structure is preferablycapable of redirecting in a viewing direction light emitted from thesaid first light-emissive region towards the barrier structure. Suchlight could be emitted directly towards the barrier structure or couldbe waveguided to the barrier structure.

The barrier structure preferably comprises an electrode for injectingelectrical charge into the first light-emissive region.

The barrier structure preferably comprises an electrically insulatingformation (which may be light-transmissive) and a light-reflectivelayer. The light-reflective layer is preferably formed over an uppersurface of the insulating formation, which surface is preferably shapedto support the light-reflective layer in a configuration in which it iscapable of absorbing light that reaches it from the first light-emissiveregion or reflecting such light out of the device in a viewingdirection. The light-reflective layer may be provided by an electrode ofthe device, preferably the cathode.

The electrically insulating formation suitably defines wells forreceiving the light-emissive material during formation of the device.The light-emissive material may be deposited into these wells by ink-jetprinting.

The light-emissive structure may comprise an electrode for injectingcharge into the first light-emissive region, suitably the anodeelectrode. That electrode may be capable of guiding light emitted fromone of the light-emissive regions towards the other emissive region.

According to a second aspect of the present invention there is provideda display device comprising: a light-emissive structure formed on asubstrate and including a region of light-emissive material for emittinglight in a viewing direction; and a light-reflective structure formed onthe substrate alongside the light-emissive structure for redirecting inthe viewing direction light propagating from the light-emissive regionto the light-reflective structure.

Preferably the light-reflective structure comprises an electrode forinjecting electrical charge into the light-emissive material, mostpreferably the cathode electrode.

The barrier structure preferably comprises an electrically insulatingformation and a light-reflective layer as described above.

The light-emissive material is suitably an organic material andpreferably a polymer material. The light-emissive material is preferablya semiconductive and/or conjugated polymer material. Alternatively thelight-emissive material could be of other types, for example sublimedsmall molecule films.

One or more charge-transport layers may be provided between eachlight-emissive region and one or both of the electrodes.

Each light-emissive region suitably represents a pixel of the display.

In one preferred embodiment the barrier structure consists of orcomprises a transparent or translucent dome-shaped or otherwise profiledregion (suitably of electrically insulating material) located around theedges of the emissive region. This may be defined by the intersection ofa metallic cathode and a transparent anode, which reflects, refracts,waveguides or otherwise directs the light emerging from the edge of theemissive pixel re-directing it towards the viewer. The dome-shaped orotherwise profiled region which re-directs the light is suitably locatedaround the periphery of the emissive region in a position such that thelight emerging from the edge of the emissive layer is deflected by meanssuch as a mirror or waveguide or both towards the viewer. The emissiveregion is suitably not covered by this profiled structure, allowing thetransparent electrode to be coated, in the window defined by theprofiled structure, with one or more organic materials that form theemissive region. The profiled surface of the structure can be coatedwith a reflective conducting metallic mirror which may be the same layeras that used for a pixel cathode. The profiled structure once formed ispreferably thermally stable and will preferably not deform or outgas onsubsequent processing during fabrication or during storage or hightemperature operation.

One preferred way for the barrier structure to inhibit the propagationof light from one of the light-emissive regions to the other is for saidstructure to act as a mirror, so that radiation is redirected, either toleave the device in the viewing direction, or else back towards thepixel from which it was emitted (from where it may be directed towardsthe viewer, by scattering or other processes). There are many methodsknown within the art for achieving such reflecting properties, and theseinclude the use of metallic mirrors and also the use of dielectricstructure in which the reflective properties are achieved by ensuringthat propagating optical modes are forbidden (as for example is achievedwith dielectric stack mirrors).

In another preferred embodiment the barrier structure consists of orcomprises a portion of the emissive layer that extends slightly beyondthe emissive region as defined by the overlap or intersection of thecathode and anode and has profiled and reflectively coated edges suchthat the light being waveguided within the emissive layer can bereflected, refracted, waveguided or otherwise directed into a directiontowards the viewer. Thus, in this embodiment, rather than forming thelight re-directing structure from a different material the organicelectroluminescent material itself is profiled. This is potentially asimpler structure in design and may have advantages in that it may beeasier and therefore cheaper to fabricate.

In another preferred embodiment a substrate which has one or moreregions coated with a transparent conductor forming isolated pixelanodes is spin coated with a layer of photoresist which is formed intoseparate islands using standard photolithographic techniques. Theseseparate islands are suitably located above the anodes in areas wherethe emissive regions will be formed and may have a square or rectangularfootprint. The photoresist preferably extends laterally beyond the edgesof these emissive regions and preferably has a profile with steep sidewalls. The substrate may then be heated to allow the photoresist islandsto adopt a hemispherical or more generally a rounded profile due tosurface tension. The photoresist islands may then be coated with areflective material such as a metal to form mirrors which are conformantto the profile of the photoresist islands, and are preferably curved.The central portion of the rounded structure may then be removed, e.g.using standard photolithographic techniques. The reflective metalliclayer may act as a mask and/or etch stop for the plasma etching of acentral window in the photoresist. Thus a window, e.g. of square orrectangular shape, can be formed leaving a curved profiled border aroundthe emissive region. The substrate can then be coated with an organicelectroluminescent material and a cathode.

In another preferred embodiment the preferably square or rectangularislands can be formed with their central portion already removed at thefirst photolithographic stage. This creates a hollow frame-likestructure around the emissive region with steep side walls on its insideand outside. After the heating stage the frame-like structure becomesmore rounded and forms a bead around the edge of the emissive area witha hemispherical or more generally rounded cross-section profile. Thiscan then be coated with an organic electroluminescent material followedby a reflective metallic coating which can also act as the cathode.

In another preferred embodiment the emissive layer is an organicelectroluminescent polymer which is coated directly onto the patternedtransparent conductor coated substrate and converted if necessary.Separate square or rectangular islands of photoresist with steep sidewalls can be formed using standard photolithographic methods. The entiresubstrate may then be subjected to plasma etching to remove both thepolymer and the photoresist to some extent. This suitably results in thepolymer layer having an angled side wall profile. A reflective coatingwhich also acts as the cathode can also be deposited on the substrate.

In another preferred embodiment a substrate coated with a patternedtransparent conductor is then coated with an insulating material such asan insulating polymer (such as polyimide) which can be patterned andprocessed using standard photolithography and is thus preferablyinsoluble in solvents that dissolve photoresist. The substrate can thenbe coated with photoresist and patterned as in the second embodiment tocreate a hollow frame-like structure to lie around the emissive region,with steep side walls on the inside and outside. Plasma etching can thenremove both the photoresist and the underlying polymer in such a manneras to produce a hollow frame-like structure in the underlying polymerwith angled side walls. This structure can then be coated with anorganic electroluminescent material and a reflective metallic coatingwhich also acts as the cathode.

Some preferred materials for components (where present) of the displaydevice are as follows:

One of the electrodes (the hole-injecting electrode) preferably has awork function of greater than 4.3 eV. That layer may comprise a metallicoxide such as indium-tin oxide (“ITO”) or tin oxide (“TO”) or a highwork function metal such as Au or Pt. The other electrode (theelectron-injecting electrode) preferably has a work function less than3.5 eV. That layer may suitably be made of a metal with a low workfunction (Ca, Ba, Yb, Sm, Li etc.) or an alloy or multi-layer structurecomprising one or more of such metals together optionally with othermetals (e.g. Al). At least one of the electrode layers is suitably lighttransmissive, and preferably transparent, suitably at the frequency oflight emission from one or more of the light-emissive regions.

The or each charge transport layer may suitably comprise one or morepolymers such as polystyrene sulphonic acid doped polyethylenedioxythiophene (“PEDOT-PSS”),poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoicacid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (“BFA”),polyaniline and PPV.

The or each organic light-emissive material may comprise one or moreindividual organic materials, suitably polymers, preferably fully orpartially conjugated polymers. Suitable materials include one or more ofthe following in any combination: poly(p-phenylenevinylene) (“PPV”),poly(2-methoxy-5(2′-ethyl)hexyloxyphenylenevinylene) (“MEH-PPV”), one ormore PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives),polyfluorenes and/or co-polymers incorporating polyfluorene segments,PPVs and related co-polymers,poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)(“TFB”),poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene))(“PFM”),poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene))(“PFMO”), poly (2,7-(9,9-di-n-octylfluorene) (“F8”) or(2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (“F8BT”).Alternative materials include small molecule materials such as Alq3.

According to a third aspect of the present invention there is provided amethod for forming a display device of any of the types described above.

According to a fourth aspect of the present invention there is providedelectronic apparatus (such as a computer display or a portable computer)comprising a display device of any of the types described above.

Any implied orientation of the device is not necessarily related to itsorientation during use or manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 3 shows cross-sections illustrating successive stages in theformation of a first device;

FIG. 4 shows a cross-section of a second device;

FIG. 5 shows cross-sections illustrating successive stages in theformation of a third device;

FIG. 6 shows cross-sections illustrating successive stages in theformation of a fourth device;

FIG. 7 shows cross-sections illustrating successive stages in theformation of a fifth device;

FIG. 8 is a partial cross-section of a fifth device;

FIG. 9 is an enlarged view of part of FIG. 8; and

FIG. 10 is an enlarged photograph of part of the device of FIG. 7 fromthe principal viewing direction, normal to the plane of the device..

The figures are not to scale.

DETAILED DESCRIPTION

The substantially completed structure of the device of FIG. 3 is shownin FIG. 3(d). The device comprises a transparent substrate 20 on whichthere is an insulating region 21. The inner walls 22 of the insulatingregion define a well in which light-emissive region 23 can be depositedby ink-jetting. Electrodes 24, 25 are located on either side of thelight-emissive region and are connected to driving circuitry 26 whichallows a suitable voltage to be applied across the light-emissive regionin order for it to emit light. A light-reflective layer 27 lies over theupper/side walls 28 of the insulating region 21, and its lower surfaceconforms to the shape of the upper/side walls. The shape of the lowersurface of the light-reflective layer is such as to cause at least somelight that is emitted into the device (rather than directly outwardstowards a viewer 29) to be directed out of the device in a viewingdirection. For example, the paths of some modes that are reflected inthis way by the light-reflective layer are shown at 30 and 31. Theeffect of the light-reflective layer is thus to increase the proportionof the light that is emitted from the light-emissive region 23 thatexits the device in viewing directions. This increases the effectivepower efficiency of the device. The device of FIG. 3 could be extendedto comprises more than one light-emissive region, as shown in FIG. 4. Inthis device the light-reflective layer not only increases the effectivepower efficiency of the device, but can also inhibit cross-talk andincrease contrast between the light-emissive regions. This latter effectcould also be achieved if the light-reflective layer were replaced witha light-absorbent layer.

FIGS. 3(a) and 3(c) show successive stages in the formation of thestructure shown in FIG. 3(d). The substrate 20 is a glass sheet, and iscoated with transparent conductive indium-tin oxide (ITO) which formsthe anode electrode 24. Such ITO-coated glass substrates arecommercially available. The ITO is patterned to create the required formof the anode. A layer of positive photoresist 21 is then spin-coatedover the ITO and the glass, and then patterned using standardphotolithographic techniques of masking during exposure to ultra-violetradiation, development, baking, etc., to form an island of photoresistover the electrode 24. The photoresist covers the electrode and extendssideways a little beyond the edge of the electrode. The photoresistcould, for example, be between a few and a few tens of microns thick.This results in the structure of FIG. 3(a).

The structure is then heated so as to melt or at least soften thephotoresist and cause it to bead up under the action of surface tension(tending to reduce the surface area) to form the generally semicircularcross-sectional shape illustrated in FIG. 3(b). The footprint of thephotoresist on the substrate (which could for example be round, squareor rectangular) is maintained by the strong interaction between thephotoresist ant the substrate material. This interaction could beincreased if necessary by using an adhesion promoter, which could bespun on to the substrate immediately before the photoresist is applied.

A highly reflective material such as aluminium is then deposited overthe photoresist and the adjacent area of the glass substrate to form thereflective layer 27. The aluminium may be deposited using DC magnetronsputtering to a depth of around 100 nm. A second layer of positivephotoresist is deposited over the aluminium and using the same masking,exposure, development and baking etc. steps as before a window is formedin the photoresist at the location over the electrode 24 where thelight-emissive material is to be deposited. This window extends throughthe full depth of the photoresist and is preferably slightly smaller inarea, although the same shape, as the electrode 24. Then the aluminium27 is under the window is removed using reactive ion etching in anatmosphere of silicon tetrachloride. This forms a window in thealuminium of the same shape as the one in the second photoresist layer.An anisotropic plasma etching step is then carried out in an oxygenatmosphere, which removes the second layer of photoresist and, under thewindow formed in the aluminium layer 27, the first layer of photoresist21. The aluminium layer 27 acts as a mask for the etching of thephotoresist 21. The photoresist 21 under the window in the aluminium 27is removed to the depth of the electrode 24. This forms the structureillustrated in FIG. 3(c), where the well has a rectangular footprint outof the plane of the figure, steep side walls and is around 2 μm deep.

A layer of light-emissive polymer such as PPV is then ink-jet printedinto the well to a final thickness of around 100 nm. This can beachieved by ink-jet depositing a PPV precursor polymer that is a randomcopolymer with acetate side-groups and tetrahydrothiophenium leavinggroups with bromide counter ions in a water/methanol mixture. Thisstructure is then baked at 150° C. for 4 hours in a high-purity nitrogenatmosphere to convert the PPV precursor into the electroluminescentcopolymer film of thickness approximately 100 nm and to remove thesolvents. The electroluminescent PPV copolymer layer 23 that is formedhas conjugated PPV segments and non-conjugated α-acetyloxy,p-xylyleneunits. A cathode electrode 25 is then deposited over the light-emissivepolymer layer 23 by DC magnetron sputtering. A suitable material for thesputtering target is 95% Al, 2.5% Lt, 1.5% Cu, 0.5% Mg. This producesthe structure shown in FIG. 3(d).

Contacts are then attached to the electrodes and the device can beencapsulated, for example in an epoxy encapsulant, for environmentalprotection.

When the device is in use, light that is emitted in all but extremelateral directions towards the glass substrate will pass directly out ofthe device. (This is illustrated for a point in the emissive region byrange of angles A). Light that is emitted sideways (range of angles B)hits the light-reflective layer 27 and is reflected by that layertowards the glass substrate and thus passes indirectly out of thedevice.

The profiled structure is preferably sufficiently inert that it does notcause problems by, for example, outgassing once the device has beenformed.

The device of FIG. 4 is formed in a similar way to that of FIG. 3. Likeparts are numbered in FIG. 4 as in FIG. 3. The device of FIG. 4 mayprovide a two-dimensional orthogonal array of light-emissive regions.The anode electrodes 24 in the device of FIG. 4 may connectlight-emissive regions in a single row of the array and the cathodeelectrodes 25 of FIG. 4 may connect light-emissive regions in a singlecolumn of the array. By this means the display driver can address thelight-emissive regions using a passive matrix addressing scheme.Alternatively, the electrodes could be configured, and if necessary thedisplay provided with extra circuitry (e.g. thin-film transistor (TFT))circuitry, to allow the display driver to use an active matrixaddressing scheme.

FIG. 5 illustrates an alternative device structure. In this structurethe well for receiving the light-emissive material is defined by a wall40 of photoresist that surrounds the region where light-emissivematerial 41 is to be deposited (FIG. 5(a)). This wall of photoresist isthen heated to round its shape. (FIG. 5(b)). In this embodiment thecathode 42 itself is used as the light-reflective layer (FIG. 5(c)). InFIG. 5 the glass substrate is shown at 43 and the light-emissive layerat 44.

FIG. 6 illustrates another alternative structure in which the edge ofthe emissive material 50 itself is shaped so that the conformant cathodelayer 51 of reflective material can overly the emissive material and actas a light-reflective layer. In this embodiment the light-emissivematerial overlaps the edge of the anode 52 so that there is not anelectrical short between the electrodes. The glass substrate is shown at53.

FIG. 7 illustrates another alternative structure. In this embodiment theanode layer 60 is formed on the glass substrate 61 as described above.Over the anode layer and the glass substrate an insulating layer 62 ofpolyimide is formed. Over the polyimide layer a layer of positivephotoresist is deposited and this is patterned using the processdescribed above to leave a region 63 of photoresist that covers thepolyimide around the outer edge of the anode 60. (See FIG. 7(a)). Thenthe polyimide is removed in the regions where it is not protected by thephotoresist to leave a bank 64 of polyimide whose inner walls 65 definethe well that is to receive light-emissive material and which overlapsthe edge of the anode 60. (See FIG. 7(b)). Then the light-emissivematerial 66 and the light-reflective cathode 67 are deposited as before.In this embodiment of the bank 64 also insulates the anode 60 from thecathode 67. The lower surface of the cathode conforms to the surface ofthe bank. The outer walls of the bank are angled so as to allow thereflective cathode to reflect sideways-emitted light out of the device.

FIG. 8 shows a cross-section of part of another structure. In thisstructure there is a glass substrate 70 on which is deposited TFT activematrix circuitry 71. Over the TFT circuitry an insulating layer 72 ofSiO2 is deposited and a contact via 73 is made through it to meet theoutput of the TFT circuitry. On the top of the SiO2 an ITO anode layer74 is formed. The aluminium 75 is deposited so as to overlap the edge ofthe ITO and also fill the via 73 so as to contact the ITO to the outputof the TFT circuitry. Over the ITO and the aluminium are formed banks ofSiO2 76 which define a well for containing a charge transport layer 77(in this case of PEDOT:PSS) and over it a layer 78 of light-emissivematerial (in this case of a mixture of 5F8BT with TFB). Over that acathode 79 is formed.

The refractive index of the layers 78, 77, 74 and 72 are around 1.7,1.4-1.65, 1.7-1.8 and 1.54 respectively. When light is emitted sidewaysin this device some is reflected off the aluminium 75 back into theemissive layer (see arrow X in FIG. 9, which shows an enlarged view ofpart of the device of FIG. 8) but some modes are believed to bedeflected out of the device in viewing directions (see arrow Y in FIG.9).

FIG. 10 shows the emission from an array of such devices. The directemission from the light-emissive region is shown at 80 and the indirectemission after deflection believed to be by the aluminium 75 is shown at81. The cross-section of FIG. 8 is on part of the line A—A in FIG. 10.

Individual devices of the types shown in FIGS. 5, 6 and 7 could, ofcourse, be combined into multi-device structures. Additional layers suchas charge transport layers could be used in any of the devices.

The principles described above could be applied to other types oforganic or inorganic display devices. One specific alternative exampleis the class of display devices that use sublimed molecular films forlight emission, as described for example in “Organic ElectroluminescentDiodes”, C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913-915(1987).

The applicant draws attention to the fact that the present invention mayinclude any feature or combination of features disclosed herein eitherimplicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any of the present claims. In view of theforegoing description it will be evident to a person skilled in the artthat various modifications may be made within the scope of theinvention.

What is claimed is:
 1. A display device comprising: a light-emissivestructure including two regions of light-emissive material for emittinglight in a viewing direction, the regions being spaced apart in adirection perpendicular to the viewing direction and the light-emissivestructure being capable of guiding light emitted from one of thelight-emissive regions towards the other light-emissive region; and abarrier structure located between the light-emissive regions forinhibiting the propagation of light guided from the said one of thelight-emissive regions to the other light-emissive region, wherein thebarrier structure is an electrode for injecting electrical charge intothe said one of the light-emissive regions.
 2. Electronic apparatuscomprising a display device as claimed in claim
 1. 3. A display deviceas claimed in claim 1, wherein the light-emissive material is an organiclight-emissive material.
 4. A display device as claimed in claim 1,wherein the light-emissive material is a semiconductive polymermaterial.
 5. A display device as claimed in claim 1, wherein thelight-emissive material is deposited by ink-jet printing.
 6. A displaydevice comprising: a light-emissive structure formed on a substrate andincluding a region of light-emissive material for emitting light in aviewing direction; and a light-reflective structure formed on thesubstrate alongside the light-emissive structure for redirecting in theviewing direction light propagating from the light-emissive region tothe light-reflective structure, wherein the light-reflective structureis an electrode for injecting electrical charge into the light-emissivematerial.
 7. A display device as claimed in claim 6, wherein thelight-emissive material is an organic light-emissive material.
 8. Adisplay device as claimed in claim 6, wherein the light-emissivematerial is a semiconductive polymer material.
 9. A display device asclaimed in claim 6, wherein the light-emissive material is deposited byink-jet printing.